Nice article from Dr Johnson of Koivet :

Saprolegnia (or sap) is a freshwater fungus which simply means it lives in fresh water environments and needs water to grow and reproduce. Sap can also be found in brackish water and moist soil. It is often referred to as a “cold water” fungus as it flourishes in colder water, but it lives well in a wide range of water temperatures extending from 37 deg F to 91 deg F (3 to 31 deg C). While it has been described also as a “mold”, sap is a genus of fungus, with the difference being a “mold” is a mass of fungi.

The following is a list of conditions that support the spread of sap:

Overcrowding – stress and too many organics in too little water
Handling – stress and removal of the mucous coat on the fish
Epidermal integrity – open wounds that provide direct access to tissue
Parasites and pathogens – parasites cause wounds that allow pathogens (like bacteria) to enter thetissue thus giving sap a change to take hold and stress
Pollution – stress and reduced water quality
Spawning – stress and physical damage
Water quality – stress and reduced physiological conditions
Water temperature changes – stress.

Treatment :
Malachite Green (MG): Hands down, this is the best treatment for all fungal problems, including sap. MG baths and dips work_ exceptionally well and one treatment will usually solve the problem (assuming any contributing causes are corrected as well). There are certain cautions that go with using MG, such as use only in cooler, well aerated water, so read the label carefully for instructions and precautions. MG is not recommended as a pond-wide treatment for sap as the concentration levels needed to kill off sap are too high for a pond-wide treatment to be effective. Such products as ProForm C, a Malachite Green-formalin combination can help reduce the incidence of sap and parasites in the water, but because the sap hyphae may be deeply embedded, the stronger dip or bath method is recommended. Caution: Malachite green is carcinogenic and has been banned for use on food fish. Use MG with great care.